This invention relates to reducing the electrochemical corrosion potential of components exposed to high-temperature water. More particularly, this invention relates to the application of catalytic nanoparticles to high temperature water systems to obtain improved protection from corrosion and intergranular stress corrosion cracking (IGSCC).
Nuclear reactors are used in central-station electric power generation, research, and propulsion. A reactor pressure vessel contains the reactor coolant, typically water, which removes heat from the nuclear core. Piping circuits carry the heated water or steam to steam generators or turbines and carry circulated water or feedwater back to the reactor vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288xc2x0 C. for a boiling water reactor (BWR), and about 15 MPa and 320xc2x0 C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental, and radiation conditions.
The materials exposed to high-temperature water in a nuclear reactor include carbon steel, alloy steel, stainless steel, nickel-based alloys, cobalt-based alloys, and zirconium-based alloys. Despite careful selection and treatment of these materials, corrosion occurs when the materials are exposed to the high-temperature reactor water. Such corrosion contributes to a host of problems, including stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves, and buildup of gamma radiation-emitting isotopes such as Co-60.
Stress corrosion cracking (SCC) is one phenomenon that is known to occur in reactor components that are exposed to high-temperature water. Such components include structural members, piping, fasteners, and welds. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing acting in combination with corrosion at the crack tip. These reactor components are subject to a variety of stresses associated with differences in thermal expansion, operating pressures needed for containment of the reactor cooling water, residual stresses from welding, cold working, and other asymmetric metal treatments. Water chemistry, welding, heat treatment, and radiation may also increase the susceptibility of a metal or alloy component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water. SCC is further increased in the presence of a high radiation flux, which produces oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals by radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion potential, which is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces, is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes a small fraction of the water to decompose, forming H2, H2O2, O2 and oxidizing and reducing radicals. At steady-state operating conditions, equilibrium concentrations of O2, H2O 2, and H2 are established in both the recirculated water and the steam going to the turbine. The O2 and H2O2 generated by radiolysis are oxidizing species and produce conditions that can promote intergranular stress corrosion cracking (IGSCC) of materials within the BWR. One method of mitigating IGSCC of susceptible material is through the application of hydrogen water chemistry (HWC), in which hydrogen gas is added to the reactor feedwater. Hydrogen addition, by producing a more reducing condition in the reactor feedwater, has the effect of altering the oxidizing nature of the BWR environment. When the added hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species to yield water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rates of these recombination reactions are dependent on local radiation fields, water flow rates, and other variables.
By reducing the level of oxidizing species, such as dissolved oxygen, in the water, the injected hydrogen also lowers the ECP of metals that are exposed to the high temperature water. Other factors, however, such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation, result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen may be needed to sufficiently reduce the level of oxidizing species and maintain the ECP below a critical potential required for mitigation of IGSCC in high-temperature water. As used herein, the term xe2x80x9ccritical potentialxe2x80x9d denotes an electrochemical corrosion potential at or below a range of values between about xe2x88x920.230 and about xe2x88x920.300 V based on the standard hydrogen electrode (SHE) scale. When the ECP of a system is greater than the critical potential, IGSCC proceeds at an accelerated rate. Conversely, IGSCC proceeds at a substantially lower rate when the ECP of a system is less than the critical potential. The ECP of metals, when exposed to water that contains oxidizing species such as oxygen, increases to a value above the critical potential, whereas the ECP will remain below the critical potential when the metal is exposed to water containing low levels of oxidizing species.
Electrochemical corrosion potentials of stainless steels in contact with reactor water containing oxidizing species can be reduced below the critical potential of some stainless steel components by injecting a quantity of hydrogen into the water that is sufficient to produce a dissolved hydrogen concentration of between about 50 and 100 ppb or greater. Conditions necessary to inhibit IGSCC can be established in certain locations of the reactor with adequate rates of hydrogen addition to the feedwater. Different locations in the reactor system require different levels of hydrogen addition. Much higher hydrogen injection levels are necessary to reduce the ECP within the high radiation flux of the reactor core, or when oxidizing cationic impurities, such as cupric ions, are present.
The IGSCC of 304 stainless steel (e.g., composition in weight % 18.0-20.0 Cr, 8.0-10.0 Ni, 2.00 Mn, 1.0 Si, 0.08 C, 0.08 S, 0.045 P) commonly used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below xe2x88x920.230 V(SHE). Hydrogen water chemistry (HWC) is an effective method of achieving this objective. However, the large amounts (e.g., at least about 100 ppb) of hydrogen that may be required to reduce the ECP below the critical potential can result in the production of short-lived N-16 species in the steam, which in turn produces a higher radiation level in the steam-driven turbine section of the BWR. For most BWRs, the amount of hydrogen that must be added to mitigate IGSCC of pressure vessel internal components results in an increase in the main steam line radiation monitor by a factor of five. This increase in main steam line radiation can cause high environmental dose rates that may require expensive investments in shielding and radiation exposure control. Thus, recent investigations have focused on obtaining the benefits of HWC while minimizing levels of hydrogen addition and, consequently, the main steam radiation dose rates.
U.S. Pat. No. 5,135,709 to Andresen et al. discloses a method for lowering the ECP on components formed from carbon steel, alloy steel, stainless steel, nickel-based alloys, and cobalt-based alloys that are exposed to high-temperature water. According to this method, a catalytic layer of a platinum group metal is formed on the surface of the component(s). As used herein, the term xe2x80x9chigh temperature waterxe2x80x9d denotes water having a temperature of about 100xc2x0 C. or greater, steam, or the condensate thereof, and the term xe2x80x9ccatalytic layerxe2x80x9d means a coating on a substrate, or a solute in an alloy formed into the substrate, the coating or solute being capable of catalyzing the reaction of oxidizing and reducing species on the surface of the substrate. As used herein, the terms xe2x80x9cplatinum group metalxe2x80x9d or xe2x80x9cnoble metalxe2x80x9d denote metals selected from the group consisting of platinum, palladium, osmium, ruthenium, iridium, rhodium, and mixtures thereof.
One effective approach to lowering the ECP of reactor components is to either coat or alloy the stainless steel surface with palladium or any other platinum group metal. The presence of palladium on the stainless steel surface reduces the hydrogen needed to reach the required IGSCC critical potential of about xe2x88x920.230 V(SHE). To date, the palladium coating has been deposited by electroplating, electroless plating, plasma deposition, and related high-vacuum techniques, and palladium alloying has been carried out using standard alloy preparation techniques. Both of these approaches to platinum metal incorporation are ex situ techniques: they cannot be practiced during reactor operation. Thus, to be appropriately serviced, the reactor must be shut down. Reactor downtime is extremely expensive and extensive planning is needed to safely service the reactor during shutdown periods.
Accordingly, what is needed an improved process of protecting the components from corrosion and intergranular stress corrosion cracking, while eliminating the need for expensive and complicated reactor shutdowns. The present invention seeks to satisfy that need.
The present invention fulfills these and other needs by providing a method and a system for reducing the electrochemical corrosion potential of components exposed to high temperature water.
Accordingly, one aspect of the invention is to provide a method of reducing the electrochemical corrosion potential of a component exposed to high temperature water in a hot water system in which the presence of at least one oxidizing species in the high temperature water raises the electrochemical corrosion potential of the component. The method comprises the steps of: providing a reducing species to the high temperature water; providing a plurality of catalytic nanoparticles to the high temperature water, wherein each of the catalytic nanoparticles provides a catalytic surface on which the reducing species reacts with the at least one oxidizing species present in the high temperature water; and reacting the reducing species with the at least one oxidizing species on the catalytic surface and reducing the concentration of the at least one oxidizing species that is present within the high temperature water, thereby reducing the electrochemical corrosion potential of the component.
A second aspect of the present invention is to provide a method of providing at least one noble metal to a hot water system for catalyzing the formation of water from hydrogen and oxygen present within high temperature water in the hot water system. The method comprises the step of providing a plurality of noble metal nanoparticles to the high temperature water during operation of the hot water system, wherein the plurality of noble metal nanoparticles has a mean particle size of up to about 100 nm.
A third aspect of the present invention is to provide a method of reducing the electrochemical corrosion potential of a component exposed to high temperature water in a hot water system in which the presence of at least one oxidizing species in the high temperature water raises the electrochemical corrosion potential of the component. The method comprises the steps of: providing a reducing species to the high temperature water; providing a plurality of noble metal nanoparticles to the high temperature water during operation of the hot water system, wherein each of the plurality of noble metal nanoparticles provides a catalytic surface on which the reducing species reacts with the at least one oxidizing species present in the high temperature water, and wherein the plurality of noble metal nanoparticles has a mean particle size of up to about 100 nm; forming a colloidal suspension of the plurality of noble metal nanoparticles in the high temperature water; and reacting the reducing species with the at least one oxidizing species on the catalytic surface and reducing the concentration of the at least one oxidizing species that is present within the high temperature water, thereby reducing the electrochemical corrosion potential of the component.
A fourth aspect of the present invention is to provide a method of reducing the electrochemical corrosion potential of a component exposed to high temperature water in a nuclear reactor in which the presence of at least one oxidizing species in the high temperature water raises the electrochemical corrosion potential of the component. The method comprises the steps of: providing a reducing species to the high temperature water; providing a plurality of noble metal nanoparticles to the high temperature water within the nuclear reactor, wherein each of the plurality of noble metal nanoparticles provides a catalytic surface on which the reducing species reacts with the at least one oxidizing species present in the high temperature water, and wherein the plurality of noble metal nanoparticles has a mean particle size of up to about 100 nm; forming a colloidal suspension of the plurality of noble metal nanoparticles in the high temperature water; and reacting the reducing species with the at least one oxidizing species on the catalytic surface and reducing the concentration of the at least one oxidizing species that is present within the high temperature water, wherein the electrochemical corrosion potential of the component is reduced.
A fifth aspect of the present invention is to provide a system for reducing the electrochemical corrosion potential of a component exposed to high temperature water in a hot water system in which the presence of at least one oxidizing species in the high temperature water raises the electrochemical corrosion potential of the component. The system comprises: a reducing species, the reducing species being deliverable to the high temperature water; and a plurality of catalytic nanoparticles, the plurality of catalytic nanoparticles having a mean particle size of up to about 100 nm. The plurality of catalytic nanoparticles is deliverable to the high temperature water, wherein each of the plurality of catalytic nanoparticles provides a catalytic surface on which the reducing species reacts with the at least one oxidizing species present in the high temperature water. The reducing species reacts with the at least one oxidizing species on the catalytic surface, thereby reducing the concentration of the at least one oxidizing species that is present within the high temperature water, thereby reducing the electrochemical corrosion potential of the component.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.